1. Field of the Invention
This invention relates to an electric vehicle, and particularly to a wet type secondary battery which can prevent deterioration of the capacity of a battery to assure a long life.
2. Description of Background Art
A wet type secondary battery in which recharging is performed by a chemical action so that it can be utilized repetitively as a power source is used as an energy source for a drive motor for an electric vehicle or a power source for the starting of an internal combustion engine of a vehicle or for a light or the like.
A wet type secondary battery includes, for example, a nickel-zinc battery cell which employs nickel (Ni) for the positive electrode and employs zinc (Zn) as the negative electrode. Ordinary actions of the cell are indicated in chemical formulae 1.
______________________________________ Entire Battery Reaction 2NiOOH + Zn + H.sub.2 O ##STR1## + Electrode Reaction 2NiOOH + 2H.sub.2 O + 2e.sup.- ##STR2## - Electrode Reaction Zn + 2OH.sup.- ##STR3## .rarw. Charging Reaction, .fwdarw. Discharging Reaction ______________________________________
Further, such reactions as indicated in chemical formulae 2 occur at the last stage of charging.
______________________________________ Entire Battery Reaction No Change + Electrode Reaction 2OH.sup.-- .fwdarw. H.sub.2 O + 1.20.sub.2 .uparw. - Electrode Reaction (1) Zinc Charging Reaction ZnO + H.sub.2 O + 2e.sup.- .fwdarw. Zn + 2OH.sup.- (2) Oxygen Gas Attracting Reaction Zn + 1/20.sub.2 .uparw. .fwdarw. ZnO ______________________________________
Oxygen gas which is produced at the last stage of charging from the nickel electrode is produced due to a rise in a potential between the electrodes, and the quantity of charging electricity is decreased by an amount corresponding to the quantity of electricity flowing through the nickel electrode which is consumed for the production of oxygen gas. On the other hand, when oxygen gas produced at the nickel electrode is not absorbed by the zinc electrode, the zinc electrode accepts a charge corresponding to an amount by which the quantity of charging flows therethrough and it is overcharged over the nickel electrode. Consequently, an active substance of the zinc electrode is consumed, and the life expectancy of the wet type secondary electrode is reduced.
In order to eliminate such a disadvantage, the wet type secondary battery should be of an enclosed type so that oxygen gas produced at the nickel electrode may be absorbed by the zinc electrode. With such construction, a charging active material produced by overcharging is changed into a discharging active material, and then, charging amounts at the nickel and zinc electrodes are balanced at the same value. Consequently, a long life can be achieved.
However, even with such construction, if deposits produced at the zinc electrode grow until they reach the nickel electrode, short-circuiting in the cell occurs, and the wet type secondary battery cannot achieve its function any more.
Accordingly, in order to prevent such short-circuiting in the cell, a separator which has a predetermined physical strength to permit permeation of ions therethrough but which prevents permeation of a deposited material of zinc therethrough and which is low in gas permeability and is formed from a hydrophilic ion permeable film is disposed between the nickel electrode and the zinc electrode. Such a wet type secondary battery is disclosed, for example, in International Publication No. W084/00642 which was filed through the Patent Cooperation Treaty.
When such a wet type secondary battery as described above is mounted on an electric vehicle, the battery is disposed such that electrode plates thereof are directed in a vertical direction. With such an orientation, electrolyte is likely insufficient at upper portions of the electrode plates. However, electrolyte is excessive at the lower portions of the electrode plates.
Since the sectional area of an ion passage is insufficient at a location at which electrolyte is insufficient, the internal resistance is increased to increase a difference in potential so that production of gas upon charging is increased. However, a reaction is likely to accelerate and heat generation is high upon discharging. On the other hand, at another location at which electrolyte is excessive, the internal resistance is low and the potential difference is low, but a condition wherein the zinc electrodes are surrounded by liquid is provided and the efficiency thereof in absorbing oxygen gas from the nickel electrodes is deteriorated.
If such a difference in reaction occurs on the same electrode plate, then consumption of an active material occurs from an upper portion of the electrode which is in an active condition, and reduction of the area of reaction, that is, reduction of the capacity, occurs at an early stage, which is not preferable.
Conventionally, various electric motorcycles and electric automobiles, hereinafter referred to as "electric vehicles," have been proposed as disclosed, for example, in Japanese Patent Application No. 3-105098. The electric vehicles are superior to vehicles on which internal combustion engines are mounted with regard to the level of exhaust gas and so forth and compatibility with diversification of energy resources.
In such electric vehicles, it is necessary to charge the battery mounted thereon. As one of charging methods therefor, there is a method wherein charging is performed using commercial power supplied from a domestic plug socket. With such charging method, a charger for charging is either carried on a vehicle or installed separately. See FIG. 34.
FIG. 32 is a block diagram showing the construction of a vehicle-carried charger. Referring to FIG. 32, a vehicle-carried charger 5108 includes a transformer 5103 including a plug 5110 to be connected to a domestic plug socket, a rectifier 5104, a current regulator 5105 and a charging current determiner means 5102. The charging current determiner means 5102 obtains information concerning the battery voltage, the battery temperature and so forth of the battery 5101, determines a charging current value in accordance with a predetermined technique and outputs this determination to the current regulator 5105 to control the current regulator 5105.
FIG. 33 is a block diagram showing construction of a separately installed charger. Referring to FIG. 33, similar reference characters to those of FIG. 32 denote similar or equivalent portions. Charging current determiner means 5106 of the separately installed charger 5108A detects, by way of a pair of power source cables 5107, a battery voltage of the battery 5101 mounted on an electric vehicle 5109. The charging current determiner means 5106 determines a charging current value from the detected battery voltage and controls the current regulator 5105 with the charging current value.
With such charging methods, the following problems occur. When a charger is mounted on a vehicle, the weight of the charger is a dead weight during traveling. Thus, there is a possibility that the charger may be damaged during the traveling of the vehicle. A vehicle-mounted charger must be selected taking its weight into consideration. Consequently, the current regulating capacity of the charger is restricted. As a result, the charging time is increased and the convenience of use is restricted.
While a separately installed charger does not have such a defect as a vehicle-mounted charger, the judgment of the completion of the charging of a battery is made by detection of a battery voltage which is performed by way of a power source cable interconnecting the battery and the charger or by management of a charging time. This system cannot accurately fully charge the battery due to a temperature variation in the battery or a voltage drop by the power source cable. Further, since a separately installed charger is manufactured in accordance with an electric vehicle to be used, it cannot be used for charging when the type of the battery such as lead, Ni-Cd, Ni-Zn or the like or the charging method is different or the voltage is different due to a difference of a cell.
For such a charging method, a system has been proposed wherein a charging station similar to a gasoline station adapted for internal combustion engine vehicles is installed and charging is performed by the charging station. An example of such charging station is shown in FIG. 34.
Referring to FIG. 34, a charging station 5111 has the function of connecting an electric vehicle 5201 thereto using a power source cable 5112 and supplying at least a charging current. In addition, the station has another function of measuring, in accordance with the necessity, the charged power and displaying a fee and so forth on a display panel 5111A.
Such a charging station 5111 is likely used to assure a cruising distance of a return path on the way of traveling of the electronic vehicle, for example, on the way of commuting or shopping, and so forth. Accordingly, it is desirable to employ a charger which is short in charging time and, in other words, can take a high charging current.
However, a charger of such a high current is not suitable as a vehicle-carried charger as described hereinabove with reference to FIG. 32. In addition, a charging method and/or a charging current are different for different types of electric vehicles. Thus, it is not suitable to provide a charger on a charging station side intended for use together with various electric vehicles.
The traveling distance of an electric vehicle relies upon the battery capacity. A battery of a large capacity is employed for an electric vehicle. However, if the battery is increased in size and the weight of the vehicle is increased, then the traveling distance is decreased as a result. Therefore, an expensive battery which is small in size but has a large capacity is employed as a battery for an electric vehicle. Accordingly, if a problem occurs with any one of the separators so that one of cells of the battery cannot be used any more, not only a heavy burden is economically imposed on a user but also heavy labor is required for exchanging the battery.
In view of such regards as described above, there is a tendency that, as a battery for an electric vehicle, a so-called assembly battery is utilized which is constituted from a plurality of small batteries, battery cells, which are electrically connected in series. If such an assembly battery is employed, in case only one of the cells is not available, it is only necessary to exchange the battery cell, and accordingly, the economic burden and the labor required for maintenance can be restricted to be a minimum.